My Volvo S40 wont pass Emissions because the check engine lite don't turn on.

Yes sir... & i made a mistake on the year... Im really sorry, it's a 2001 Volvo S40...

That's OK! There probably isn't a huge difference anyways. Check the instrument cluster when you turn the key on with the engine off. All your warning lights, including the check engine light, should be on. If not, the gauge cluster will need to be inspected to find the fault that prevents the light from working, then address any other issues.

Thank you very much for your time, & i will check the cluster. A friend of mines said that there's a fuse thats in that plug where they plug in to get the codes. I'm not sure weather to believe that or not.. But i will most definitely check the cluster... I will respond when i find out if or not that is the problem...

Sounds good. I've never heard of a fused diagnostic connector either. Nothing wrong with checking the fuses, but they are probably all good.

Well, for now all the dash lights turn on minus the the check engine light... I haven't checked the cluster just yet but I will... It got late on me & it is cold... So i will get back to you... Again I thank you for your time... Well, today I took off the dash instruments & to my disbelief, i found the problem!!! The guy that i bought this car from rolled up 4 balls of black tape & stuffed them in to the holes where the bulbs are because the check engine light was on & in order to sell the car he had to make it look like the car was running great. I wanted to get mad but i couldnt stop laughing. I took a pictureof it & i sent it to my friend who is also a Great Machanic & who told me to check the bulb. Well, so i did & that was what I found... I also found that this guy, had also removed 5 more light bulbs & i need to find the plastics that the bulbs sit on so that i can put bulbs on them & install them in there proper place... But, because of your advice I found the problem... Therefore, I thank you, & my daughter thanks you... Thank you for your time in helping me find the issues on my daughters volvo...

Excellent! I'm so glad you figured it out, and if you happen to report the guy that sold you the car, don't expect too much to come of it. However, that is an awesome find!! As for the p0420, likely catalytic converter or oxygen sensor failure (probly the catalytic converter). The catalytic converter ha's to match the engine family code stamped on the catalyst information sticker under the hood. Again, I'm very glad you worked it out, and the othe person that posted here (which is why I saw your last message) said his was $700. That may have